1. Field of the Invention
The present invention generally relates to a developing device for use in an electrophotographic copying machine, a printer or the like and more particularly, to a developing device which is capable of steadily providing a uniformly formed thin layer of charged toner with respect to an electrostatic latent image formed on the surface of a photosensitive member or photoreceptor of the copying machine or the like.
2. Description of the related Art
In an electrophotographic or electrostatic copying machine, the surface of a photoreceptor which is of an image support member is preliminarily electrically charged uniformly and is, then, exposed to light on the basis of a pattern corresponding to an image of an original document so that a latent image may be formed thereon. Subsequently, upon supply of the charged toner onto the surface of the photoreceptor having thereon the latent image formed by a developing device, the latent image is developed into a visible toner image and thereafter, the toner image obtained is transferred onto and fixed on a transfer sheet or copy paper sheet.
In the developing device for use in such electrophotographic copying machine or the like, particularly, in the developing device employing therein non-magnetic toner as one-component developer, it is especially important to supply a uniformly formed thin layer of the charged toner onto the surface of the photoreceptor.
Conventionally, U.S. Pat. No. 4,100,884 discloses one of such developing devices, in which upon supply of the non-magnetic toner onto the surface of an elastic developing roller, a blade is pressed against the developing roller so that a thin layer of charged toner may be formed on the peripheral surface thereof and a toner image is, then, formed by bringing the thin layer of the charged toner into direct contact with the surface of the photoreceptor.
In this kind of the developing device, however, to form the thin layer of the charged toner, it is necessary to keep the blade in contact with the surface of the developing roller under a certain pressure greater than a predetermined one. To this end, the developing roller is requested to be relatively high in hardness. On the contrary, it is desirable for the developing roller to be as soft as possible to prevent the photoreceptor from being damaged or the image from being broken on a contact portion between the developing roller and the photoreceptor.
From an above-mentioned point of view, Japanese Patent Laid-open Application No. 55-77764 discloses another developing device in which an electrically conductive thin film is arranged on the surface of a developing roller of an electrically conductive soft elastic foamed member. In this developing device, the toner is caused to electrically adhere to the surface of the developing roller with the use of a magnetic brush and the developing is, the, executed by causing the toner to adhere to an electrostatic latent image through contact between the developing roller and the surface of the photoreceptor.
However, even when the developing roller employing therein the foamed material is used, the pressure between the developing roller and the photoreceptor can not be sufficiently lowered. In particular, in the case where a peripheral speed of the developing roller is to be differentiated from that of the photoreceptor, the image formed on the photoreceptor tends to be disadvantageously spoiled.
Namely, when the pressing force onto the developing roller becomes greater, the force operating in the direction of a sleeve rotation causes the image formed on the photoreceptor to be broken, e.g., cracking and/or deformation of the image, and fogging becomes so much. On one hand, it is not practical to set the pressing force weakened since it becomes impossible that the developing roller is brought into contact with the photoreceptor with uniformly and steadily nipping in a direction along the width of the photoreceptor.